1. Field of the Invention
This invention relates to sewing machines in general and more particularly to a photoelectric detecting system for detecting the depletion of bobbin thread.
2. Description of the Prior Art
The use of photoelectric detectors for detecting the exhaustion of bobbin thread is well known in the prior art. See for example U.S. Pat. No. 3,845,320 to Winberg, U.S. Pat. No. 2,350,397 to Haas, and U.S. Pat. No 3,631,901 to Langenbach. One problem associated with prior known bobbin thread run out detectors is that they are sensitive to stray light in the area occupied by the photodetector. Moreover, the prior art bobbin thread run out detectors are not suitable for detecting the rate at which bobbin thread is being consumed. Another problem of many prior art bobbin thread run out detectors is the need to locate the photodetector on the opposite side of the bobbin from the light source, thereby occupying much of already limited space in the vicinity of the hook mechanism. Still another problem with some prior art bobbin thread detectors is that the characteristics of light reflected from the bobbin to the photodetector do not change significantly as the thread supply is consumed, and therefore these prior detectors are overly sensitive and unreliable.